Installatron has been running without issue for many months on a server with multiple sites.
Today a single site reported an error while attempting to upgrade.
The backup failed with error:
Could not connect to `localhost` The server requested authentication method unknown to the client

All other sites appear to be functioning properly.

Google indicates the issue is with "mysql_old_password authentication method"

I'm unclear how this can only be affecting a single site and no others on the server.

I'm not sure why but the password in MySql was indeed using a old 16-byte password and not a newer 41-byte password format.
I simply updated the password ( to the same password ) and it stored it in 41-byte format.